Method and apparatus and screen for screening mechanical fiber pulp

ABSTRACT

The invention relates to a method, an apparatus and a screen for screening mechanical fibre pulp. According to the invention a short fibre material is separated using a gap screen, whereby said material can be very efficiently separated to form a specific fraction.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation of International Application No. PCT/FI01/00896, filed Oct. 16, 2001, which designated the U.S. and was published under PCT Article 21(2) in English, which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a method for screening a mechanically manufactured fibre pulp mixture containing fibres of varying lengths, in which method the fibre pulp mixture obtained by mechanically defibering wood in the presence of water is screened into at least two fractions containing fibres of mainly varying lengths.

[0004] The invention also relates to an apparatus for screening a mechanically manufactured fibre pulp mixture containing fibres of varying lengths, the apparatus comprising screening means for screening the fibre pulp mixture obtained by mechanically defibering wood in the presence of water into at least two fractions containing fibres of mainly varying lengths.

[0005] The invention further relates to a screen for screening a mechanically manufactured fibre pulp mixture containing fibres of varying lengths into two fractions containing fibres of substantially varying lengths, the screen comprising at least one screening surface through which water is removed from the fibre pulp mixture and some of the shorter fibres with the water and the remaining fibre pulp mixture is removed from the screen from the same side of the screening surface to which the fibre pulp mixture was fed.

[0006] 2. Description of Related Art

[0007] Mechanical fibre pulp is manufactured by grinding or thermomechanically refining wood into fibre in the presence of water, resulting in a fibre pulp mixture. Fibres having different freeness values are typically created in prior art manufacturing processes. The division of the fibres is based on a defibering process and the material used, and the ratio between the fibres having different freeness values cannot be significantly adjusted during the actual defibration. The result is therefore a fibre pulp mixture, in which the portion of fibres having different freeness values and operating properties is by no means necessarily the most favourable as regards use. After this the fibre material is screened in the prior art methods using screens comprising various screening surfaces so that fibres, which are smaller than a particular size and which have passed through a screening surface, are directed for use, and the coarser fibres are removed from the process and returned again to the process through reject refining. However, the final result is a fibre pulp mixture, where the lengths of the fibres and thus their freeness values vary widely, and the structure of the fibre pulp mixture and the properties thereof do not correspond with the desired ones. When making a particular type of paper, it is preferable to use such mechanical fibre pulp that contains long fibres of high quality and short fibres provided with a binding ability, but not substantially mid-size fibres. Such fibre pulp cannot be achieved using conventional mechanical fibre pulp screening methods.

BRIEF SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a method, an apparatus and a screen that enable to achieve fibre fractions containing fibres of mainly varying lengths to be appropriately used for different purposes.

[0009] The method of the invention is characterized in that in at least one screening phase the fibre pulp mixture or a part thereof is screened to separate short fibres from the fibre pulp mixture or from a part thereof using a gap screen comprising a convergent gap and at least one wire restricting said gap, whereby the fibre pulp mixture to be screened or a part thereof is fed into said convergent gap so that it flows in the same direction as the wire towards an output port at the convergent end of the gap, and the short fibres and some of the water fed into the gap exit through the openings in the wire whereas the water and some of the remaining fibre pulp mixture exit through the output port of the gap.

[0010] The apparatus of the invention is characterized in that the apparatus comprises a gap screen to separate short fibres comprising a convergent gap and at least one wire restricting said convergent gap, whereby the fibre pulp mixture to be screened or a part thereof is fed into said convergent gap so that it flows through the gap screen in the same direction as the wire towards the convergent end of the gap so that the short fibres and some of the water exit through the openings in the wire and the remaining part of the fibre pulp mixture exits from the gap screen through the output port at the convergent end of the gap.

[0011] The screen of the invention is characterized in that the screen is a gap screen comprising a convergent gap restricted by a wire and into which the fibre pulp mixture is fed to move in the same direction as the wire, whereby water and shorter fibres exit through the openings in the wire and the remaining fibre pulp mixture exits from the output port at the convergent end of the gap.

[0012] An essential idea of the invention is that an extremely fine and short-fibered fibre pulp material is separated from the fibre pulp mixture using a gap screen, preferably one formed by means of twin wires, whereby a very short-fibred material having a high freeness value is obtained that cannot be so distinctly distinguished as a specific pulp by means of other screening methods. In accordance with a preferred embodiment of the invention, a part of a fibre pulp mixture containing medium-length and shorter fibre materials preferably already screened by separating long fibres therefrom is screened anew using said gap screen formed by means of twin wires. According to a second preferred embodiment of the invention, the medium-sized fibre material containing some fine fibres separated from the gap screen is directed further to be refined. As a result a fraction containing medium-sized and short fibres with the desired properties is obtained.

[0013] The invention provides such an advantage that a fibre fraction that is shorter than a fraction representing a particular length can be very accurately separated from the fibre pulp mixture containing fibres of varying lengths. An advantage provided by a preferred embodiment of the invention is that as the long-fibred fibre material is at first screened and thereafter a very short fibre material having a high freeness value is separated using the gap screen from the remaining fraction containing medium-sized and short-fibred material, then the final result obtained is three fibre pulp fractions with different properties that can be mixed with one another or with other fibre materials in order to achieve a desired fibre web quality. Hence, long fibres can be combined with very short fibres in order to obtain a particular paper quality, which is not possible when medium-sized fibres are included in the fibre pulp.

[0014] Consequently, fibre pulp fractions of varying properties and lengths can be obtained as desired from the mechanically manufactured fibre pulp, and thereby the desired fractions can easily be combined with the manufacturing of fibre web, and only the fibre pulp fractions required can be used to produce a particular type of fibre web. Thus, a fibre web that contains only long or short fibres can be manufactured and the medium-sized fibre obtained can be used to produce another type of fibre web.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0015] In the following the invention is described in greater detail in the appended drawings, in which

[0016]FIG. 1 schematically shows the method and apparatus of the invention,

[0017]FIG. 2 schematically shows a gap screen to be used in the application of the method of the invention, and

[0018]FIG. 3 schematically shows a screening result obtained using the gap screen of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019]FIG. 1 schematically shows the method and apparatus of the invention, in which freeness values of a particular fibre pulp are provided by way of example, on the basis of which values the division into different fibre pulp fractions can be carried out.

[0020]FIG. 1 schematically shows how a material 1 to be defibered is directed to a defibrator 2, which may be a grinder known per se or a refiner, typically a thermomechanical pulp refiner. The fibre pulp mixture obtained from defibration is directed to screening means, or at first to a first screen 3, where the mixture can be divided for example in accordance with freeness value “16” so that the fraction containing long fibres and has passed through the first screen 3 proceeds to be further processed and the fraction containing even longer fibres and shives and has not passed through said screen is directed to be refined to a first recirculation refiner 4, from where the fraction is returned to the first screen 3 for screening. Consequently, a fibre that is longer than the desired one continuously circulates until it passes the first screen and then continues onwards in the procedure. The first refiner may either be a separate refiner, or the fraction to be returned can also be returned to the actual defibrator 2 as indicted by the dashed line.

[0021] The fibre pulp fraction containing long fibres and has passed through the first screen is directed forward to a second screen 5, where the fraction is screened again according to length on the basis of the fibre length provided by way of example with freeness value “28”. The fibre pulp fraction that has passed through the second screen 5 continues forward to be further processed and the longer fibre pulp fraction that is unable to pass through said screen is directed to a second refiner 6, which is preferably a refiner known per se. The fibre pulp obtained from the second refiner 6 is directed to a third screen 7, from where the fibre that is shorter than a particular length and that has passed through said screen, for example a fibre provided with freeness value “40” is directed forward for use, and correspondingly the fibre that is unable to pass through the screen 7 is returned again to the second refiner 6 to be refined. Hence, the long fibre circulates the same loop until it is shortened to a desired size in such a manner that it passes through the third screen 7 and is then ready for use. The fibre pulp fraction that has passed through the second screen 5 is directed in accordance with the invention to a fine screening 8, where a fine fibre pulp fraction is separated from the fibre pulp using a separate gap screen, for example fibres provided with a freeness value of at least “200” and correspondingly fibres that are longer than that are directed to a third refiner 9, which preferably is a refiner known per se. The fibre pulp fraction obtained from the output of the third refiner 9 contains high-quality fines and medium-sized fraction.

[0022] The gap screen is either a single wire or twin wire structured screen. Such a screen can be used to efficiently remove a desired amount of fibre pulp of a particular length or shorter than that from the fibre pulp mixture, thus making the division between the fractions drastic and providing a desired ratio. The other screens are screens known as such either so that each screen represents a separate individual screen or that two or more screens form one multi-phase screen. Then again it is necessary to use a gap screen to separate short fibres.

[0023]FIG. 2 schematically shows gap a twin wire structured screen 10. The gap screen preferably allows to screen the fibre pulp into two fractions in accordance with the invention. The gap screen comprises two movable wires 11 and 12 forming opposing screening surfaces that move through the gap screen and return as closed loops so as to form a convergent gap 13 within the gap screen in the direction of movement thereof. Supporting structures 14 and 15 are placed at the back of the wires 11 and 12, i.e. on the opposite side of the gap 13. The wires 11 and 12 are supported against the supporting structures 14 and 15, which naturally include openings for receiving water and the fine fibre material removed through the openings of the wires 11 and 12 and removing the same from the gap screen as schematically shown by arrows A. The wires 11 and 12 circulate around rollers 16 and 17 shown by way of example, which as such is known in the art. The sides of the gap 13 at the edges of the wires 11 and 12 are naturally closed, so that the fibre pulp mixture can be fed from a feeding channel 18 to the gap screen appropriately under pressure so as to remove the water and fine fibre material as desired. What is also schematically indicated in the Figure is engines 19 and 20, which are connected to rotate one of the rollers 16, and 17 respectively, although in a normal situation the engines can be used to circulate each roller separately or a required amount of the rollers in a manner known as such in the art. The engines 19 and 20 are, in turn, connected to a control unit 21 included in the control means that allows to adjust, for example, the rotational speed of the engines so as to make the wires 11 and 12 move at a desired pace, which may deviate from the flowing rate of the fibre pulp through the screen and most preferably exceeds the flowing rate of the fibre pulp mixture.

[0024] During the screening the fibre pulp mixture is fed from the feeding channel 18 to the gap screen, and the mixture thus flows through the gap 13 of the screen and exits from a narrower output port 22 of the gap 13, i.e. the smaller output port in cross section as regards the feeding opening, from the same side of the screen surfaces or the wires, to which the fibre pulp mixture was originally fed. At the same time, owing to the structure of the wires 11 and 12, water is removed from the openings in the wires as well as short fibre material. Depending on the amount of fibre material to be removed the speed of motion in the wires can be adjusted in relation to the feeding rate of the fibre pulp mixture fed into the screen until the desired screening result is obtained. At the same time, more water can be removed from the fibre pulp mixture with the short fibre pulp fraction in comparison with the amount of water that remains with the fibre pulp fraction that does not pass through the wires, i.e. exits through the output port of the gap 13. Consequently the longer fibre pulp fraction can simultaneously also be thickened, and the consistency may rise up to 10%. In this application and claims the output port 22 refers to the part in the screen, where water and fibres are no longer removed through the wire or wires and the remaining fibre pulp mixture is transferred from the screening phase to the following possible processing phases either directly or controlled by various conveyor means and control means such as wires and the like.

[0025] What can naturally also affect the quality of the fraction is to appropriately select the dimensioning and penetration of the wires 11 and 12, whereby wires provided with different penetration properties can be used for different purposes. What can also affect the screening is to set one or both of the wires to move towards the gap along a convex supporting structure, in which case water and the fine fibres are at the same time removed more efficiently from the fibre pulp fraction at the feeding end of the screen, and correspondingly more slowly towards the output end of the gap screen. If the aim is to use a single wire structure, then one of the wires 11 and 12 can be replaced with a fixed wall surface for example, and a combination can preferably be employed in which the wire 11 moves towards an arched supporting structure 14 and the specific wall surface replaces the wire 12 moving directly within the area of the gap.

[0026] In practice, the adjustment of the screening is mainly based on the fact that the gap screen adjusts the basis weight of the fibre pulp mixture including the long-fibred fibre material that flows through said gap screen. This can be implemented either by adjusting the consistency of the fibre pulp mixture to be fed into the screening or the velocity of the wires in relation to the feeding rate of the fibre pulp mixture. When a diluted fibre pulp mixture is fed into the gap screen, more short fibres can be removed from the fibre pulp mixture than if a more consistent fibre pulp mixture is fed under similar circumstances. Correspondingly, if the velocity of the wires is increased in relation to the feeding rate of the fibre pulp mixture, then more short fibres can correspondingly be removed from the fibre pulp mixture. Thus, the separation value of the short fibres can be set at a desired level by adjusting the consistency of the feed and/or the velocity of the wires, so that the result obtained is distinctly different fibre pulp fractions, where the fraction that passes through the wires contains only very short fibres and correspondingly the fraction that flows through the gap of the gap screen contains longer fibres but very few short fibres. The velocity of the wires can be adjusted in various ways, so that it remains below the feeding rate of the fibre pulp, exceeds the feeding rate of the fibre pulp or equals the feeding rate of the fibre pulp. The choice of velocity is based on the properties of the fibre pulp mixture to be screened and on the desired screening result.

[0027]FIG. 3 schematically shows the result of the screening obtained by means of a gap screen shown in FIG. 1 in accordance with the method of the invention. FIG. 3 shows the screening result, or the portion of the fibres having different freeness values, in the fibre pulp mixture fed and correspondingly in the fraction that passes through the gap and exits through the output end of the gap screen. As FIG. 3 shows the portion of the fibres having different freeness values is presented in such a manner that the portion of different fibres in the fibre pulp mixture fed of the entire fibre pulp mixture is indicated by the black column and correspondingly the portion of fibres having a corresponding freeness value included in the fibre pulp fraction that exits through the output end is indicated by the white column. As FIG. 3 shows the portion of fibres having a freeness value 30 or greater than that is nearly 30%, the portion of fibres having a freeness value ranging from 50 to 30 is nearly 16%, the portion of fibres having a freeness value ranging from 100 to 50 is roughly 13%, the portion of fibres having a freeness value ranging from 200 to 100 is approximately 7% and the amount of very fine fibres having the freeness value 200 is 34%. In such a situation, the average freeness value of the fibre pulp mixture fed is 100.

[0028] The fibre pulp that has completed the screening shows that the portion of coarse fibres has risen proportionally by nearly 50%, the portion of the following fibres approximately 25%, whereas the portion of the shorter fibres has increased only slightly. The extremely short fibres have, in turn, been removed to a great extent through the wires with the water, and the portion of the short fibres has therefore decreased proportionally by 60%. In practice, this means that only the extremely short-fibred material is substantially removed in the gap screen from the fibre pulp mixture fed, the freeness value of said short-fibred material being in this example approximately 200 or higher. Hence, the extremely short fibre can efficiently be removed from the fibre pulp mixture to be screened and thus obtain a specific fibre pulp fraction that can be used for various purposes by mixing a desired amount thereof to the long fibres for example, in order to manufacture a particular type of fibre web provided with specific properties. Correspondingly the average freeness value of the fibre pulp mixture that passes through the gap screen has increased.

[0029]FIG. 3 shows that the amount of long fibres in the fraction that passes through the wires is substantially 0 and correspondingly the amount of short fibres in the long-fibred fraction that passes through the gap screen is very low. This indicates that the gap screen operates fairly efficiently and divides the fibre pulp mixture drastically into two fractions of distinctly varying lengths. An appropriate selection of wire structures in the gap screen allows to determine the applicable screening conditions for the fibres of varying lengths in accordance with use and to adjust the efficiency of the screening by controlling the velocity and/or the consistency of the fibre pulp mixture.

[0030] The invention is described above in the specification and the drawings by way of example but is not merely restricted thereto. What is essential is that mechanically manufactured fibre pulp is screened using a gap screen in such a manner that a short fibre fraction is separated therefrom and a desired amount of fibre pulp fractions of predetermined fibre lengths is obtained and preferably so that the short fibre fraction is separated from such a fibre pulp from which the long fibres have already been separated and the short fibres can therefore be as efficiently as possible separated from the rest of the fibre material. 

That which is claimed is:
 1. A method of screening a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers and shorter fibers, said method comprising: directing the fiber pulp mixture into an input end of a convergent gap of a gap screen, the convergent gap extending from the input end to an output port and being at least partially defined by at least one wire screen capable of moving therebetween; and moving the at least one wire screen toward the output port so as to direct the fiber pulp mixture therethrough, the at least one wire screen defining a plurality of openings configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture, whereafter a remainder of the fiber pulp mixture is discharged through the output port.
 2. A method according to claim 1 wherein the convergent gap is defined by two opposed wire screens and moving the at least one wire screen further comprises moving the two opposed wire screens toward the output port, each wire screen defining a plurality of openings configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings.
 3. A method according to claim 1 wherein directing the fiber pulp mixture into the input end further comprises directing the fiber pulp mixture into the input end at a feed rate.
 4. A method according to claim 3 wherein moving the at least one wire screen toward the output port further comprises moving the at least one wire screen toward the output port at a rate of motion, the rate of motion being adjustable in relation to the feed rate of the fiber pulp mixture.
 5. A method according to claim 1 further comprising adjusting a consistency of the fiber pulp mixture prior to directing the fiber pulp mixture into the input end.
 6. A process of screening a fiber pulp mixture, including longer fibers and shorter fibers, using a first and a second gap screen, each gap screen including a convergent gap having an input end and an output port and being at least partially defined by at least one wire screen capable of moving therebetween, said process comprising: directing the fiber pulp mixture into the input end of the convergent gap of the first gap screen; moving the at least one wire screen toward the output port of the first gap screen so as to direct the fiber pulp mixture therethrough, the at least one wire screen defining a plurality of openings configured such that a first portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings, the first gap screen thereby separating the first portion of the fiber pulp mixture including shorter fibers from the fiber pulp mixture, whereafter a first remainder of the fiber pulp mixture is discharged through the output port of the first gap screen; directing the first portion of the fiber pulp mixture including shorter fibers into the input end of the convergent gap of the second gap screen; and moving the at least one wire screen toward the output port of the second gap screen so as to direct the first portion of the fiber pulp mixture including shorter fibers therethrough, the at least one wire screen defining a plurality of openings configured such that the shorter fibers are capable of passing through the openings, the second gap screen thereby separating at least a portion the shorter fibers from the first portion of the fiber pulp mixture including shorter fibers, whereafter a second remainder of the fiber pulp mixture is discharged through the output port of the second gap screen.
 7. A method according to claim 5 further comprising directing the first remainder of the fiber pulp mixture through a first refiner so as to produce a first refined fiber pulp mixture.
 8. A method according to claim 7 further comprising directing the first refined fiber pulp mixture from the first refiner into the input end of the convergent gap of the first gap screen.
 9. A method according to claim 5 further comprising directing the second remainder of the fiber pulp mixture through a second refiner so as to produce a second refined fiber pulp mixture.
 10. A method according to claim 9 further comprising directing the second refined fiber pulp mixture from the second refiner into the input end of the convergent gap of the second gap screen.
 11. An apparatus adapted to screen a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers and shorter fibers, said apparatus comprising: a gap screen having a convergent gap, the convergent gap having an input end for receiving the fiber pulp mixture and an output port; at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the fiber pulp mixture therethrough; and a plurality of openings defined by the at least one wire screen, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture and discharging a remainder of the fiber pulp mixture from the output port.
 12. An apparatus according to claim 11 further comprising two opposed wire screens operably engaged with the gap screen and at least partially defining the convergent gap, the two opposed wire screens being configured to extend between the input end and the outlet port and to be movable toward the output port, each wire screen defining a plurality of openings configured such that the portion of the fiber pulp mixture including shorter fibers is capable of passing through the openings.
 13. An apparatus according to claim 11 wherein the fiber pulp mixture received by the input end is directed thereto at a feed rate, and wherein the apparatus further comprises a control means operably engaged with the at least one wire screen and configured to adjust a rate of motion of the at least one wire screen in relation to the feed rate of the fiber pulp mixture.
 14. A system adapted to screen a fiber pulp mixture, the fiber pulp mixture being formed by defibering wood in the presence of water and comprising longer fibers, slivers, and shorter fibers, said system comprising: a first screening apparatus, comprising: a gap screen having a convergent gap, the convergent gap having an input end for receiving the fiber pulp mixture and an output port; at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the fiber pulp mixture therethrough; and a plurality of openings defined by the at least one wire screen, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the fiber pulp mixture and discharging a first remainder of the fiber pulp mixture from the output port.
 15. A system according to claim 14 further comprising a first refiner configured to receive and refine the first remainder of the fiber pulp mixture from the output port of the first screening apparatus to produce a first refined fiber pulp mixture, the first refiner being further configured to direct the first refined fiber pulp mixture back to the input end of the first screening apparatus.
 16. A system according to claim 14 further comprising a second screening apparatus adapted to receive the portion of the fiber pulp mixture including shorter fibers from the first screening apparatus, said second screening apparatus comprising: a gap screen having a convergent gap, the convergent gap having an input end for receiving the portion of the fiber pulp mixture including shorter fibers and an output port; at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the portion of the fiber pulp mixture including shorter fibers therethrough; and a plurality of openings defined by the at least one wire screen, the openings being configured such that the shorter fibers are capable of passing therethrough, the gap screen thereby separating at least a portion of the shorter fibers from the portion of the fiber pulp mixture including shorter fibers and discharging a second remainder from the output port.
 17. A system according to claim 16 further comprising a second refiner configured to receive and refine the second remainder of the fiber pulp mixture from the output port of the second screening apparatus to produce a second refined fiber pulp mixture.
 18. A system according to claim 17 further comprising a third screening apparatus adapted to receive the second refined fiber pulp mixture from the second refiner, said third screening apparatus comprising: a gap screen having a convergent gap, the convergent gap having an input end for receiving the second refined fiber pulp mixture and an output port; at least one wire screen operably engaged with the gap screen and at least partially defining the convergent gap, the at least one wire screen extending between the input end and the output port and configured to be movable toward the output port so as to direct the second refined fiber pulp mixture therethrough; and a plurality of openings defined by the at least one wire screen, the openings being configured such that fine fibers in the second refined fiber pulp mixture are capable of passing therethrough, the gap screen thereby separating at least a portion of the fine fibers from the second refined fiber pulp mixture and discharging a third remainder from the output port to the second refiner.
 19. A system according to claim 14 further comprising a consistency control means adapted to adjust a consistency of the fiber pulp mixture prior to the fiber pulp mixture being received by the first screening apparatus.
 20. A wire screen adapted for use in a gap screen having a convergent gap with an input end and an output port, the wire screen extending between the input end and the output port, being movable toward the output port, and at least partially defining the convergent gap, the gap screen being further adapted to receive a fiber pulp mixture comprising longer fibers and shorter fibers in the input end thereof, said wire screen comprising: at least one screening surface configured to engage the fiber pulp mixture received by the gap screen; a plurality of openings defined by the at least one screening surface, the openings being configured such that a portion of the fiber pulp mixture including shorter fibers is capable of passing therethrough, the wire screen thereby being configured to separate at least a portion of the shorter fibers from the fiber pulp mixture so as to produce a remainder of the fiber pulp mixture incapable of passing through the openings in the at least one screening surface.
 21. A wire screen according to claim 20 wherein the wire screen is configured as an endless belt. 